New outline stroker from FreeType

FreeType recently received an update to its stroker, which supposedly fixes many rendering issues. And indeed it does! However, first I needed to fix the outline preprocessing in libass, since it wasn't very cleanly handling outline modifications. Now that this is fixed, here's a quick comparison of old vs. new stroker.

One font that has been very problematic for FreeType's stroker is Comiquita Sans. Previously, the outlines generated by the stroker were broken in pretty funny ways and this was clearly visible in the rendering:

The new stroker fixes these issues completely:

The new stroker is available in FreeType git master and will appear in the next FreeType release (2.4.6).